Pollution is a worldwide problem which does not respect national boundaries and is likely to intensify as the spread of industrial development continues. BBC News looks at some of the places around the world which are hardest hit by pollution.

Arctic: Persistent Organic Pollutants (POPs)

The Arctic has a severe problem with persistent organic pollutants (POPs). POPs are chemical substances which accumulate in the food chain, threatening both human and animal health as well as the environment.

According to researchers, dangerous levels of POPs have been found in the Arctic's air, snow, water and wildlife.

POPS can harm animals, especially those higher up the food chain

It is thought that POPs, like the pesticide DDT, are carried on air currents from the mid-latitudes of North America, Europe and Asia.

Once they reach the Arctic, the harsh climate causes them to freeze into the snow and ice, where they accumulate and concentrate up the food chain.

Although the Arctic might seem like a pristine and remote environment, its severe cold actually encourages POPs to enter the system.

These pollutants can harm many animals, especially those higher up the food chain. According to some researchers, they may be weakening the immune function of mammals like polar bears as well as causing reproductive problems.

The Prestige oil tanker sank near northern Spain on 19 November 2002, polluting about 3,000 km (1,800 miles) of coastline.

The spill is estimated to have killed 300,000 seabirds, making it one of Europe's worst wildlife disasters.

The economic cost of the accident to fishing and tourism has been put at about 5bn euros (£3.4bn).

The polluting effects of the Prestige oil spill could still be an issue today. Although a clean-up operation has removed most of the oil on coastal land, there are concerns about the large quantity which sank to the sea bed.

WWF says it may release contaminants which could enter the food chain, including into commercially caught species such as sea bass, octopus, shrimps and crabs.

The shrinking Aral Sea is a severe trouble spot in Kazakhstan and Uzbekistan, because of over-irrigation and pollution.

Ships have been left beached on the dry Aral sea bed

Inefficient irrigation systems still consume huge amounts of water which would once have reached the sea.

It has shrunk from a volume of about 1,000 cubic km 40 years ago to 110 cubic km today.

The mineral content of the water is now seven times higher than it was four decades ago.

The water is being severely polluted by pesticides and fertilisers, which local farmers use on their cotton crops.

Where the water has retreated completely there is a vision of environmental apocalypse - vast stretches of desert laden with heavy doses of salt and burdened with a toxic mix of chemical residues washed down over the decades from the farms upstream.

Not only has this devastated natural ecosystems in the area, it has also affected the health of the local human population.
Malnutrition is rife and conditions like anaemia and TB are increasing.

The rate of cancer of the oesophagus is higher near the Aral Sea than anywhere else in the world.

A huge "dead zone" of deoxygenated water spreads across the Gulf of Mexico every summer because of severe nitrate pollution.

Dead zone water is completely uninhabitable for most marine animals, and in the Gulf of Mexico it can cover an area of about 15,000 sq km (5,800 sq miles).

The Gulf of Mexico's dead zone has been an annual problem for the last 30 years, because farmers in the Mississippi watershed are using large quantities of nitrate-based fertilisers.

These cause an algal bloom in the water, which guzzles oxygen, suffocating other forms of marine life.

At the moment little is being done to alleviate the problem, and according to conservationists, some locals actually welcome the dead zone's arrival because crabs and lobsters are easy prey as they flee the deoxygenated water.

In January 2000, a storage pond at a gold mine near the city of Baia Mare, in northern Romania, burst its banks in what was to become known as an aquatic version of Chernobyl.

Some 100,000 cubic metres of water, containing an estimated 100 tonnes of cyanide, spilt into small local rivers, then finally into the River Tisza in nearby Hungary.

The spill wiped out all fish and plant life for several hundred kilometres. Five weeks later another spill, this time of heavy metals, struck the same region. It was a disaster for the surrounding river systems.

According to the United Nations the spill, which killed thousands of fish in Hungary and Yugoslavia, was one of the worst river pollution accidents in Europe.

Even two years after the incident fishermen in Hungary claimed their catches were only a fifth of their former levels.

China's breakneck economic growth and soaring energy demand has caused it to suffer from some major pollution problems.

At the moment about two-thirds of the country's power comes from coal and coal products - the cheapest and dirtiest forms of energy.

According to the World Bank, air pollution costs the Chinese economy $25bn a year in health expenditure and lost labour productivity - largely because of the use of coal.

This exacts a very real human toll - official figures say 400,000 Chinese citizens die a year from diseases related to air pollution, and, according to the World Bank, 16 of the 20 most polluted cities in the world are in China.

According to the Food and Agriculture Organization (FAO), over 100,000 tonnes of old and unused toxic pesticides have been abandoned in sites around Africa and the Middle East.

Leaking and corroding metal drums filled with obsolete and dangerous pesticides dot urban and rural landscapes. These chemical leftovers - including the infamous DDT, which is banned in many countries - can harm the environment as well as human health.

The scope of the problem is dramatically illustrated in Ethiopia, where some 3,400 tonnes of obsolete pesticides - some of which are over 20 years old - are stored at 1,000 sites throughout the country.

In the western Ethiopian village of Arjo, FAO researchers found over five tonnes of DDT and malathion in a collapsing barn just yards from homes and pastures.

Residents had long complained of nausea, respiratory ailments and headaches, and reported a strong stench from the unprotected site.

Cleaning up these dangerous substances will cost a huge amount of money - some estimates place it as high as $250m.